(1) Field of the Invention
The present invention relates to a digital recording/reproduction apparatus for reproducing information in a digital signal recorded on a recording medium.
(2) Description of the Prior Art
In a digital video tape recorder, there is known a partial response/class 4 as a method for digitalizing and recording a video signal on a recording medium such as a magnetic tape and the like. With this method, the video signal digitalized and recorded can be decoded using Viterbi decoding.
However, since the Viterbi decoding is for processing a digital signal, when the Viterbi decoding is performed, in order to perform quantization, it is necessary to convert the reproduction signal reproduced from the recording medium from the analog signal to the digital signal, using a sampling clock of a predetermined sampling rate.
In this case, if the phase of the sampling clock is not proper, the reproduction signal in the analog form cannot be quantized properly. Particularly, in the case of the Viterbi decoding, since it is for discriminating the decoded data and correcting errors according to the amplitude level, if the phase of the sampling clock is shifted and the amplitude level is not correctly converted, a code error occurs.
Specifically speaking, for example in FIG. 21, when the recording signal is xe2x80x9c0, 1, 0, 1, 0, 0, 1, 0xe2x80x9d, if the reproduced waveform is simply detected with the threshold of xe2x80x9c1xe2x80x9d and xe2x80x9cxe2x88x921xe2x80x9d (reproduction without Viterbi decoding), the detection result becomes xe2x80x9c0, 1, 1, 1, 0, 1, 1, 0xe2x80x9d.
On the contrary, if the reproduced waveform in FIG. 21 is detected using the Viterbi decoding, when there is a xe2x80x9cpeakxe2x80x9d where the amplitude level is higher than point B or a xe2x80x9cvalleyxe2x80x9d where it is lower than point B, after the amplitude level at point B exceeds the threshold xe2x80x9c1xe2x80x9d and from point C downward, xe2x80x9c1xe2x80x9d will be determined as the data at point B. At point C, the amplitude level is higher than the threshold xe2x80x9c1xe2x80x9d, but lower than point B, hence the data at point C is not determined.
At the next point D, the amplitude level becomes lower than xe2x80x9cxe2x88x921xe2x80x9d, and for the first time, xe2x80x9c1xe2x80x9d is determined as the data at point B, and xe2x80x9c0xe2x80x9d is determined as the data at point C. However, the data at point D is not determined at this moment. In order to determine the data at point D, it is necessary to have a xe2x80x9cvalleyxe2x80x9d where the amplitude level is lower than the threshold point D or a xe2x80x9cpeakxe2x80x9d where it is higher than xe2x80x9c1xe2x80x9d at point E or downward.
Then, at point E, since the amplitude level does not exceed xe2x80x9c1xe2x80x9d, the data at point D is not yet determined. At the next point F, since the amplitude level is not lower than the amplitude level at point D, the data at point D is not yet determined. It is not until the amplitude level at the next point G exceeds xe2x80x9c1xe2x80x9d that xe2x80x9cxe2x88x921xe2x80x9d is determined as the data at point D, xe2x80x9c0xe2x80x9d is determined as the data at point E, and xe2x80x9c0xe2x80x9d is also determined as the data at point F, respectively.
As described above, with the Viterbi decoding, decoding is performed by detecting the amplitude level of the reproduced waveform, hence the detection accuracy of the amplitude level is quite important. The amplitude level of the reproduced waveform is determined at a time position of point A and the like in the reproduced waveform shown in FIG. 21, and the time position is determined by the phase of the sampling clock signal, hence the detection accuracy of the amplitude level depends largely on the phase of the sampling clock.
For example, when the phase of the sampling clock signal is delayed, there is a case where the amplitude level at point E exceeds the threshold xe2x80x9c1xe2x80x9d. In this case, the data at point E will be erroneously detected as xe2x80x9c1xe2x80x9d, and the data at point F will be erroneously detected as xe2x80x9cxe2x88x921xe2x80x9d.
Thus, the phase adjustment of the sampling clock is quite important for enhancing the detection accuracy of the amplitude level in the reproduced waveform.
In the conventional apparatus, the phase adjustment of the sampling clock signal is performed by an operator on the production stage, and it takes time for adjusting the phase. Moreover, since the phase of the sampling clock signal is fixed to a value adjusted on the production stage, when tapes having different properties are reproduced due to the property difference between tape manufacturers or the difference between lots, the optimum phase point is shifted, causing a problem that the detection accuracy of the amplitude level in the reproduced waveform is decreased.
There is known a method in which even if kinds of magnetic tapes and properties of magnetic heads change, the optimum point of the clock phase is automatically searched using a mean value in the error rate of the reproduction data (Japanese Patent Application Laid-open Hei 2 No. 259891), as a conventional method for adjusting the phase of the sampling clock signal to the optimum point.
However, when the error rate is 10xe2x88x925, for example, an error occurs at a rate of once for every 100,000, hence it is necessary to read at least 1,000,000 data in order to accurately count the error rate. When the error rate becomes 10xe2x88x926, an error occurs at a rate of once for every 1,000,000, hence it is necessary to obtain at least 10,000,000 data.
Therefore, according to the conventional method, it is necessary to read much more data in order to adjust the sampling clock phase to the appropriate point. However, since the error rate becomes low as the sampling clock phase approaches the appropriate value, the data quantity to be read or the calculation time of the error rate increases exponentially. Accordingly, there has been such a problem that as the optimum condition is aimed, much time is required.
With a view to solving the above problems, it is an object of the present invention to provide a digital recording/reproduction apparatus which can quickly adjust the phase of the sampling clock signal for quantizing the reproduced waveform of the video signal to an optimum value.
In order to attain the above objectives, the present invention has the following construction.
The digital recording/reproduction apparatus according to the invention as set forth in the first aspect is a digital recording/reproduction apparatus which quantizes and decodes the information signal reproduced from a recording medium based on a sampling clock, and is characterized in that it includes detection means for detecting the frequency of occurrence of data contained in the information signal, and phase control means for controlling the phase of the sampling clock based on the frequency of occurrence of data detected by the detection means.
The digital recording/reproduction apparatus according to the invention as set forth in the second aspect has a structure of the digital recording/reproduction apparatus according to the first aspect, wherein the detection means includes means for comparing the information signal with the threshold and converting it into a binary format, and threshold control means for controlling the threshold based on the frequency of occurrence of either one of the two theoretical values contained in the output of the means for converting into a binary format.
The digital recording/reproduction apparatus according to the invention as set forth in the third aspect has a structure of the digital recording/reproduction apparatus according to the first aspect, wherein the phase control means controls the phase of the sampling clock based on the detection results of the detection means, so that the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the positive phase signal of the sampling clock is made maximum, or the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the negative phase signal of the sampling clock is made minimum.
The digital recording/reproduction apparatus according to the invention as set forth in the fourth aspect has a structure of the digital recording/reproduction apparatus according to the first aspect, wherein the detection means includes means for comparing the information signal with the threshold and converting it into a binary format, and threshold control means for controlling the threshold based on the frequency of occurrence of either one of the two theoretical values contained in the output of the means for converting into a binary format, the phase control means controlling the phase of the sampling clock based on the detection results of the detection means, so that the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the positive phase signal of the sampling clock is made maximum, or the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the negative phase signal of the sampling clock is made minimum.
The digital recording/reproduction apparatus according to the invention as set forth in the fifth or sixth aspect has a structure of the digital recording/reproduction apparatus according to the second or fourth aspect, wherein the threshold control means controls the threshold so that the frequency of occurrence is made minimum based on the amount of change in the frequency of occurrence of either one of the two theoretical values contained in the output of the means for converting into a binary format.
The operation of the present invention will now be described.
According to the digital recording/reproduction apparatus relating to the invention as set forth in the first aspect, the detection means detects the frequency of occurrence of data contained in the information signal reproduced from a recording medium and provides it to the phase control means. The phase control means controls the phase of the sampling clock for quantizing the information signal, based on the frequency of occurrence of data contained in the information signal. For example, attention is given to the fact that the amount of change in the frequency of occurrence of data contained in the information signal with respect to the sampling clock has a certain correlation with an error in this data, and the phase of the sampling clock is determined so that the amount of change in the frequency of occurrence of data becomes minimum.
According to the digital recording/reproduction apparatus relating to the invention as set forth in the second aspect, the means for converting into a binary format constituting the detection means compares the information signal with the threshold, and for example, when the information signal is larger than the threshold, the theoretical value is designated as 1, and when the information signal is smaller than the threshold, the theoretical value is designated as 0. The threshold control means determines the threshold based on the frequency of occurrence of the theoretical value 0 or 1. Thus, the threshold at the time of converting the information signal into a binary format is determined with the detection means.
According to the digital recording/reproduction apparatus relating to the invention as set forth in the third aspect, the phase control means controls the phase of the sampling clock based on the detection results of the detection means, so that the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the positive phase signal of the sampling clock (normally rotated clock) is made maximum, or the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the negative phase signal of the sampling clock (inversely rotated clock) is made minimum.
According to the digital recording/reproduction apparatus relating to the invention as set forth in the fourth aspect, the means for converting into a binary format constituting the detection means compares the information signal with the threshold, to convert the information signal into a binary format. The threshold control means determines the threshold based on the frequency of occurrence of the theoretical value 0 or 1 obtained by the means for converting into a binary format.
When the threshold of the detection means is determined, the phase control means controls the phase of the sampling clock, so that the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the positive phase signal of the sampling clock is made maximum, or the frequency of occurrence of data contained in the signal obtained by quantizing the information signal based on the negative phase signal of the sampling clock is made minimum.
According to the digital recording/reproduction apparatus relating to the invention as set forth in the fifth or sixth aspect, the threshold control means sets up the threshold so that the amount of change in the frequency of occurrence of data is made minimum. Therefore, when a quenching error or an insertion error has a characteristic that it approaches zero in an area having a small amount of change in the frequency of occurrence of data, the set threshold controls the frequency of occurrence of the quenching error or the insertion error, thereby it becomes the most adequate threshold for the detection means.